This invention relates to a machine tool for machining a workpiece that machines by reciprocating tools, such as taps and drills, in an axial direction while rotating them about their axis.
One example of this type of machine tool is a tapping unit for tapping a workpiece. This machine has a spindle which is rotatably provided around an axis in a housing. The spindle is reciprocated in the axial direction by a feed motor and also rotated at a low speed by a spindle motor. The spindle is equipped with a tap at its end portion to apply tapping to a workpiece. Generally, a DC motor or an AC inductive motor has been adopted as the above-mentioned spindle motor.
Concerning the above-mentioned tapping unit, miniaturization and weight reduction is desirable. However, it has also been desired that even a small-type tapping unit be able to tap a large-diameter workpiece. In case a DC motor is adopted as the above-mentioned spindle motor, a large size spindle motor is required to obtain sufficient torque to tap a large-diameter workpiece. As an inevitable consequence, the unit becomes a large system. Moreover, since the DC motor utilizing a permanent magnet includes a problem of demagnetization, it is impossible to provide a large current to get a large driving torque. On the other hand, in case an AC inductive motor is adopted, it is hard to obtain a large torque. It is, therefore, necessary to use a large size motor or reduce the speed of the motor by a reduction gear. As a result, the unit becomes a large scale unit or the revolution speed is reduced. Accordingly, the conventional tapping unit is still insufficient to meet the needs for miniaturization and a realization of high torque.